442 REPORT~1873. 



Treatment of Seivar/e. 



" Of the 189 towns and districts having systems of sewerage, 143 discharge 

 their sewage without any treatment whatever ; in 17 instances the sewage 

 is simply filtered before discharge, in 7 instances it is chemically treated, 

 and in 17 cases recourse is had to irrigation, whilst in 5 instances the 

 system of disposal includes more than one of these methods." By " simple 

 filtration " is generally meant mere straining, a method obviously insufEcient 

 for the purification of sewage. 



Certain processes for precipitating the valuable materials contained in 

 sewage were investigated by the Committee with the following results : — 



I. The PhospJmte Process of Messrs. Forbes and Price, which consists in the 

 addition to the sewage of (a) a mixture of native phosphate of alumina and 

 sulphuric acid, and (6) sufficient milk of lime to neutralize the sewage. The 

 result was that during its passage through a large tank " the suspended 

 matters were very completely deposited, and the supernatant water ran over 

 the sloping edge of the tank at its extreme end bright and clear and almost 

 odourless." It was found that the water did not putrefy, even after the lapse 

 of four months, that it contained only the merest trace of phosphoric acid, 

 no sulphuretted hydrogen, nor any nitrates nor nitrites, but that it contained 

 " as much actual ammonia as ordinary dilute London sewage, and also a certain 

 *.' amount of albumenoid ammonia." The precipitate had no offensive smell. 

 The valuable constituents of sewage, with the exception of the suspended 

 matters and the phosphoric acid, are not precipitated by this process, and 

 cannot be utilized unless the effluent water be afterwards used for irrigation, 

 in which case the milk of lime would not be added, and the clarified sewage 

 would still contain a quantity of phosphoric acid. 



" The advantage of this use of it, if it were found to answer from an 

 economical point of view, would be the deodorization of the deposit in the 

 tanks and of the sewage itself, which is certainly at present a great deside- 

 ratum, especially as regards the tanks." (Report III. pp. 185-187.) 



II. WliitthrecuVs Patent. — Experiment was made on 100 gallons of Eom- 

 ford sewage with one pound of the mixture used in this process — a mixture 

 which was stated to consist of dicalcic and monocalcic phosphate, two equi- 

 valents of the former to one of the latter, a little milk of lime being after- 

 wards added. The result was a very rapid precipitation, the supernatant 

 water remaining nearly clear and quite inoffensive. The precipitate, dried 

 at 100° C, contained as much as 3 per cent, of ammonia and a considerable 

 quantity of phosphate of lime. The supernatant water contained rather 

 more actual ammonia than the original sewage, but scarcely any organic 

 nitrogen, showing that the organic matters in solution, as well as those in 

 suspension, had been almost entirely removed by the process. This water con- 

 tained, however, a considerable quantity of phosphoric acid, which would be 

 valuable if the water were afterwards used to irrigate land ; " but, unless 

 means are devised for separating it, it would constitute a serious loss if the 

 water were thrown away." It must be added that tliis was regarded merely 

 as a preliminary experiment. 



III. Gemral Scott's Process. — This was investigated at Ealing. It con- 

 sists in the addition to the sewage, while in the sewers, of a mixture of lime 

 and clay, in the proportion of about 10 cwt. of the former and 5 cwt. of 

 the latter to 400,000 gallons of sewage. The result was a very complete 

 precipitation of the suspended matters, which were collected in tanks, tho 

 supernatant water being passed upwards through filter-bods, and discharged 



